1. Field of the Invention
The present invention relates to the treatment of aqueous effluents. The invention relates more specifically to an improvement to processes, according to the prior art, for treating such aqueous effluents. The said improvement facilitates the removal of materials in suspension and/or colloidal materials present in the treated effluents. These effluents, which are more or less concentrated suspensions, can consist in particular of processing waters, for example from the paper industry, industrial or urban waste waters, or even river waters or alternatively sludges, derived in particular from such waste waters.
2. Description of the Related Art
Processes for treating such effluents are directed towards producing, depending on the context, more concentrated effluents, liquid sludges or thickened sludges. They can also be directed towards depositing charges on a substrate; more generally, towards ridding the industrial effluents concerned of at least some of their charges. The said processes generally comprise several steps and involve large amounts of treatment reagents: chemical conditioning reagents and antifoaming agents, if necessary. These chemical conditioning reagents of the inorganic salt or synthetic organic polymer type, which are known to those skilled in the art, are used as coagulants and/or flocculents. They facilitate the aggregation of the solid particles in suspension--small-sized particles, colloidal particles--and, consequently, the subsequent separation of the solid and liquid phases by treatments which exert essentially physical actions (decantation, centrifugation, filiation, etc.). In paper manufacture, in the processing waters, they improve the aggregation and binding of the charges (such as talc, kaolin, etc.) or of the pigments (organic or inorganic) to the cellulosic substrate.
The cost of chemical reagents of this type can represent more than 50% of the running expenses of an aqueous effluent treatment plant, such as an urban waste water purification plant.
The improvement, proposed according to the present invention is advantageous, in particular, in that its use allows a substantial reduction in the amounts of treating reagents required and thus a saving of the same magnitude in the running expenses of the process.
Processes for treating aqueous effluents in which an inorganic acid is added to the said effluents in order to generate carbon dioxide therein have been described in patent applications JP-A-51,124,042 and JP-A-59,010,388. The said inorganic acid consumes, by chemical reaction, carbonates present in the effluent or added thereto, so as to generate the said carbon dioxide in situ. Document JP-A-59,010,388 describes the treatment of very basic effluents. In these effluents, the acid is added in an amount such that their pH is brought to a value of between 4 and 5.
The carbon dioxide, thus generated in situ by the action of a strong acid on an alkaline material, exerts, with reference to the physical phenomenon of flotation, a purely mechanical, beneficial action. This beneficial action is based on the action of the gas bubbles which rise to the surface: this is referred to as a flotation action. It cannot be based on an actual chemical action of the said gas, which, under the conditions specified, can only dissolve in the effluent in very small amounts.
It is moreover imagined that the beneficial action of the said gas is countered by the drawbacks present, on the one hand, in using a strong acid in the process, and, on the other hand, in introducing anions such as sulphate or chloride into the treated effluent, via the addition of the said acid.